1. Field of the Invention
The present invention relates to an integrated device in which a light emitting element and an external modulator (or an element functioning as light receiving element) are monolithically formed, and an integrated device in which a light emitting element and a light receiving element are monolithically formed.
2. Description of the Related Art
By virtue of development of optical fibers, optical communication technology has widely been applied to technical fields of high-speed, long-distance and large-capacity telecom systems, middle-distance telecom systems and short-distance data communication systems. Light emitting elements, light receiving elements and modulators for data transmission are key components for optical communication technology in these fields.
In the prior art, however, a light emitting element, a light receiving element and a modulator are generally manufactured independently as discrete semiconductor elements. Accordingly, the cost for manufacturing one optical system becomes enormous since time and labor is needed for the assembly and wiring of each semiconductor element.
For example, consideration will now be given of the case of using a semiconductor laser (e.g. laser diode) including a modulation function as a light emitting element.
When such a direct-modulation type semiconductor laser element is used, a transmitter unit needs to comprise an optical connector for coupling a laser beam output from the semiconductor laser to an optical fiber, a monitoring light receiving element (e.g. PIN photodiode) for stabilizing laser intensity, and an arrangement for coupling a laser beam from the opposite side of the semiconductor laser to the monitoring light receiving element.
A receiver unit is also needed in addition to the transmitter unit. In the prior art, however, the transmitter unit and receiver unit cannot share a single optical fiber, for the following reasons.
a) The receiver unit generally includes a PIN photodiode (light receiving element). Since the PIN photodiode is formed on the major surface of a semiconductor substrate, a light signal must be made incident on the major surface of the semiconductor substrate perpendicularly. By contrast, in the case of a laser diode (light emitting element), a light signal is generally emitted from a surface perpendicular to the major surface of a semiconductor substrate, i.e. a cleaved surface formed by cleaving the semiconductor substrate. PA1 b) The PIN photodiode is a device rendered operative by application of an electric field, whereas the laser diode is a device rendered operative by injection of current. It is difficult, therefore, to use a common drive circuit both for driving the PIN photodiode and for modulating the laser diode. PA1 a semiconductor substrate; PA1 a light emitting element of a surface emission type, provided on a first major surface of the semi-conductor substrate, the light emitting element radiating light towards the semiconductor substrate; and PA1 an external modulator formed on a second major surface of the semiconductor substrate and situated in a region opposed to the light emitting element. PA1 a semiconductor substrate; PA1 a plurality of surface emission type light emitting elements, provided on a first major surface of the semiconductor substrate, the light emitting elements radiating light towards the semiconductor substrate; and PA1 a plurality of external modulators formed on a second major surface of the semiconductor substrate and situated in regions opposed to the light emitting elements. PA1 a light emitting element having a radiation mode in which light is emitted from all over the grating; PA1 an external modulator situated on a light emission side of the light emitting element at a region where the light has a highest intensity; and PA1 a layer, provided between the light emitting element and the external modulator, for electrically isolating the light emitting element and the external modulator and passing the light output from the light emitting element. PA1 a light emitting element including a waveguide and having a guided mode in which light is emitted from end portions of the waveguide path; PA1 an external modulator for controlling cut/transmission (ON/OFF) of the light; PA1 a reflection mirror for guiding the light from the light emitting element to the external modulator and PA1 a layer, provided between the light emitting element and the external modulator, for electrically isolating the light emitting element and the external modulator and passing the light output from the light emitting element. PA1 a semiconductor substrate; PA1 a light emitting element of a surface emission type, provided on a first major surface of the semiconductor substrate, the light emitting element radiating light in directions towards and away from the semiconductor substrate; and PA1 a reflection mirror, provided on a second major surface of the semiconductor substrate, for reflecting the light radiated towards the semiconductor substrate. PA1 a semiconductor substrate; PA1 a light emitting element of a surface emission type, provided on a first major surface of the semiconductor substrate, the light emitting element radiating light in directions towards and away from the semiconductor substrate; and PA1 a light receiving element provided on a second major surface of the semiconductor substrate and situated in a region faced to the light emitting element.
Recently, a surface emitting type laser has been proposed, wherein a cavity is formed perpendicular to the surface of a semiconductor substrate and a laser beam is emitted from the surface of the semiconductor substrate. In the surface emitting laser, however, an electric current is injected at high density in an active layer of a small volume and the laser tends to generate heat, resulting in low output power. Thus, this laser has not yet been put to practical use.
On the other hand, a DFB (Distribution Feedback) laser is used as a light emitting element in a transmitter for high-speed, long-distance systems. The DFB laser oscillates in a single longitudinal mode, unlike an FP (Fabry-Perot) laser oscillating in a multi-longitudinal mode.
The DFB laser has a grating formed along the cavity direction. The grating has a predetermined period which determines the wavelength of the single longitudinal mode.
A DBR (Distributed Bragg Reflector) laser, like the DFB laser, is a device making use of a grating. The principle of oscillation in the single longitudinal mode of the DBR laser is similar to that of oscillation in the single longitudinal mode of the DFB laser.
Thus, in the DBR laser, too, the waveform of a signal, which has passed through an optical fiber with a wavelength distribution, does not change, and the signal can be transmitted over a long distance.
However, the modulation bandwidth of an optical output from the directly modulated DFB laser is limited by an interaction between carriers produced by current and photons generated by carries and is close to the bandwidth limit. In addition, even if the DFB laser is operated at a narrow single spectral line, complex dynamic characteristics and instability may lead to a problem of broadening the oscillation linewidth (wavelength chirp).
Accordingly, the direct-modulation type DFB laser is not suitable for much longer distance signal transmission.
In order to overcome the above problems, attention has been paid to systems using an external modulator. In this method, a laser diode is driven by a DC current. The modulation is not performed in the laser diode, and output light of the laser diode is modulated by the external modulator.
The external modulator may be of Mach-Zender type, EA type (Electro-Absorption type), etc. Recently, EA type external modulators are predominant, which permit monolithic integration of laser diodes with the external modulators.
Some specific conventional devices will now be described with reference to prior-art documents.
FIG. 1 shows a device disclosed in Document 1 (K. Wakita, et al., IEEE Photonics Technology Letter, vol. 5, No. 8, p. 899, 1993).
In this device, a DFB laser and an EA modulator are integrated monolithically. The DFB laser and EA modulator are connected coaxially and a laser beam from the DFB laser is emitted in parallel to the surface of the substrate via the modulator.
FIG. 2 shows a device disclosed in Document 2 (I. Kotani, et al., IEEE Photonics Technology Letter, vol. 5, No. 1, p. 62, 1993).
In this device, too, a DFB laser and an EA modulator are integrated monolithically. Like the device shown in FIG. 1, the DFB laser and EA modulator are connected coaxially and a laser beam from the DFB laser is emitted in parallel to the surface of the semiconductor substrate via the modulator.
FIGS. 3A to 3E show a device disclosed in Document 3 (M. Aoki, et al., Electronics Letters, vol. 27, No. 23, p. 621, 2138, 1991).
In this device, too, a DFB laser and an EA modulator are integrated monolithically. Like the device shown in FIG. 1, the DFB laser and EA modulator are connected coaxially and a laser beam from the DFB laser is emitted in parallel to the surface of the semiconductor substrate via the modulator.
FIG. 4 shows a device disclosed in Document 4 (U. Koren, et al., Electronics Letters, vol. 23, No. 12, p. 621, 1987).
This device is a discrete semiconductor device in which an EA modulator is formed monolithically.
The devices of Documents 1 to 3 are characterized in that the DFB laser and EA modulator are integrated coaxially in the direction of the waveguide.
The DFB laser having an active layer of an MQW (Multi-Quantum Well) structure has a driving electrode, to which a DC current is supplied to emit an output beam. The output beam is guided as a waveguide-mode beam to a waveguide in a modulation region. In the modulation region, only a layer necessary for guiding waves is formed, and an active layer and a grating are not provided.
If a reverse bias voltage is applied to the modulation region, a field effect, e.g. Stark effect or Franz-Keldysh effect, occurs, and an absorption band of the waveguide is shifted to the longer wavelength side. As a result, the output light of the modulator is greatly attenuated, which implies the modulation by applying voltage. In addition, a very high speed operation higher than 10 Gbps is theoretically enabled, with extremely small chirp.
However, in order to achieve the above ideal condition, electrical isolation must be maintained between the laser diode and external modulator. Furthermore, the optical return from the first and/or the modulator to the laser must be minimized to avoid the occurrence of chirp and instability of the laser.
The electrical isolation can be effected by increasing a physical distance between the laser diode and the external modulator. If the physical distance between the laser diode and external modulator is increased, the area of the chip would increase, and the optical coupling between the two devices becomes small.
To minimize the optical return to the laser is very difficult. In general, in DFB lasers and DBR lasers, the phase of light traveling reciprocally within the resonator varies due to residual reflection. Consequently, the oscillation wavelength of the laser and the intensity of output would fluctuate and in a worst case, the oscillation becomes unstable.
In conventional devices, in general, a laser beam is emitted from the laser diode in parallel to the surface of the semiconductor substrate. Specifically, when a laser diode and an external modulator are monolithically integrated, the laser diode and external modulator are connected coaxially.
This being the case, it is difficult to use planar techniques in the process of manufacturing such devices, and the manufacture thereof is difficult.